This invention is in the field of mass detection using a coated piezoelectric sensor.
In recent years there has been a growing interest in coated piezoelectric crystals, not only as highly sensitive and selective detector of various air pollutants but also as simple, inexpensive and portable device. The high sensitivity and simple relationship between mass and frequency make the quartz crystal microbalance as an ideal tool for the study of adsorption, and as a selective chemical sensor in many applications.
The principle of the detection is that the frequency of vibration of an oscillating crystal is decreased by the adsorption of a foreign material on its surface. A gaseous pollutant is selectively adsorbed by a coating on the crystal surface, thereby increasing the weight of the crystal and decreasing the frequency of vibration. The decrease in the frequency is proportional to the increase in weight due to the presence of gas adsorbed on the coating according to the following equation: xcex94F=Kxc2x7xcex94C. Here, xcex94F is the frequency change (Hz), K is a constant which refers to the basic frequency of the quartz plate, area coated, and a factor to convert the weight of injected gas (g) into concentration (ppm), and xcex94C is concentration (ppm) of sample gas.
U.S. Pat. No. 3,164,004 teaches that a piezoelectric quartz crystal coated with a substrate selectively sensitive to changes in the atmospheric environment can serve as a detection device in fluid analyzers. In general, this discovery is based on the principle that the oscillation of a crystal, both in frequency and amplitude, is in part a function of its weight. The change in weight of a crystal coated with a substrate selectively sensitive to a particular contaminant when placed in an environment containing that contaminant is, in turn, at least partly a function of the concentration of the contaminant. Therefore, the measurement of the change in oscillation characteristics of a coated crystal sensitive to a particular contaminant upon exposure to a given atmosphere is a direct and highly sensitive measure of the presence and concentration of that contaminant. Variations of and improvements in this basic method are shown, inter alia, in the following publications U.S. Pat. Nos. 5,177,994; 5,817,921, and 6,085,576; Japanese Patents Nos. 1244335, and 5187986; European Patent No. 992768, and xe2x80x9cElectronic Nose and Artificial neural Networksxe2x80x9d, L. Moy and M. Collins, American Chemical Society, Anal. Chem., 1986, 58, pp. 3077-3084; xe2x80x9cPiezoelectric Crystal Sensor for the Determination of Formaldehyde in Airxe2x80x9d, Talanta, Vol. 38, No. 5, pp. 541-545, 1991; xe2x80x9cOdor Sensing System Using Neural Network Pattern Recognitionxe2x80x9d, Toyosaka Moriizumi and Takamichi Nakamoto, International Conference on Industrial Electronics, Control, Instrumentation and Automation, Nov. 9-13, 1992, Marriot Mission Valley, San Diego, USA.
A sensor has two equally important requirements: sensitivity and selectivity. There are two ways of achieving high selectivity and specificity towards xenobiotic (non-self) agents as we can learn from nature: (i) the immune system, in which a unique sensor (i.e. antibody) is being synthesized for any invader (i.e. antigen). This is a very complicated mechanism that involves a spontaneous constant synthesis of new molecules that are examined to fit the antigen; (ii) the olfactory system, in which a huge array of receptors are located in the nose in such a way that a molecule entering the nose interacts with some of the receptors; the brain then translates the pattern of the signals to an odor. In this case the odor can be a single molecule or a composition of several different molecules.
The combination of a number of sensors and a pattern recognition routine is known as an xe2x80x9celectronic nosexe2x80x9d. Using the combination of chemical sensors, which produce a fingerprint of the vapor or gas, the recognition algorithms can identify and/or quantify the analytes of interest. The electronic nose is capable of recognizing unknown chemical analytes, odors, and vapors. In practice, an electronic nose is presented with a substance such as an odor or vapor, and the sensor converts the input of the substance into a response, such as an electrical response. The response is then compared to known responses that have been stored previously. By comparing the unique chemical signature of an unknown substance to xe2x80x9csignaturesxe2x80x9d of known substances, the unknown analyte can be determined. A variety of sensors can be used in electronic noses that respond to various classes of gases and odors.
A wide variety of commercial applications are available for electronic noses including, but not limited to, detection of explosives or drugs, environmental toxicology, biomedicine, such as microorganism classification or detection, material quality control, food and agricultural product monitoring, ambient air monitoring, employee protection, emissions control, and product quality testing. Referring to the detection of explosives, a number of laboratory techniques for the detection of explosives are known, using gas chromatography, mass spectrometry, ion mobility spectroscopy, NMR, plasma chromatography and visible chromatography. While some of these techniques are capable of ppb detection, the detection systems need elaborate techniques for operation, are usually not portable and simple, and are thus not useful for field use.
The electronic noses of today are not sensitive enough and are not very versatile, and may in some instances require a very large number of sensors and data power, making them rather expensive and slow.
Thus, there is still a need in the art for a sensing device that is compact, capable of detecting trace amounts of mass of 10xe2x88x9215 g order and even less, that is useful in a broad variety of applications and can respond accurately to a broad variety of gases, analytes, odors and fluids.
The present invention provides a sensing device (also referred to as an electronic-nose device) that is compact and, in certain embodiments, configured to be a handheld device. The device can be used to measure or identify one or more analytes in a medium such as vapor, liquid or gas.
The main idea of the present invention is based on the use of a piezoelectric crystal element (such as quartz, ceramics), which, on the one hand, can be manufactured by the existing techniques to be mountable in a sensor device, and, on the other hand, has a sufficiently thin (membrane-like) region, which defines a sensing region, thereby providing increased sensitivity as compared to the known sensors.
It is known that the less the thickness of a piezoelectric quartz crystal, the higher the sensitivity of the sensor device. The crystal element, according to the present invention, is in the form of the so-called xe2x80x9cinverted mesa structurexe2x80x9d, in which a relatively thin membrane-like region (resonator) is surrounded by relatively thick end regions of the structure. This membrane-like region is provided with a pair of electrodes. The electrodes and/or the membrane-like region have a surface region coated with reactive molecules, thereby presenting a sensing region of the device electrically excitable by the environment (which can be gas or liquid). The reactive coating molecules interact with a foreign material from the environment to yield a reaction product that effects a change in the resonance frequency of the crystal resonator from a certain resonance frequency value characterizing said crystal resonator.
The reactive coating molecules are preferably organized in a self-assembled monolayer (SAM) formed on the surface of the electrodes and/or the membrane-like region (i.e. on the surface of a substrate). The monolayer consists of receptor compounds comprising a linker that connects said compound to the surface of said substrate, an optional spacer, a structural element and an active head group.
There is thus provided according to one aspect of the present invention, a piezoelectric crystal element for use in a sensor device for identifying at least one foreign material from environment, the crystal element comprising at least one crystal resonator in the form of an inverted mesa structure having a membrane-like region and being characterized by a certain resonance frequency value, a surface region of said at least one crystal resonator being modified by reactive molecules of a kind capable of interacting with said at least one foreign material to yield a reaction product that effects a change in the resonance frequency of said crystal resonator from said certain resonance frequency value, said change being indicative of the identity and quantity of said at least one foreign material.
According to another aspect of the present invention, there is provided a piezoelectric sensor for use in a device for identifying at least one foreign material from environment, the sensor comprising a piezoelectric crystal element having at least one crystal resonator in the form of an inverted mesa structure defining a sensing membrane-like region and being characterized by a certain resonance frequency value, and electrodes formed on opposite sides of said sensing membrane-like region, said at least one crystal resonator having a surface region modified with molecules capable of interacting with at least one foreign material from the environment to which the crystal resonator is exposed to yield a reaction product that effects a change in the resonance frequency of said at least one crystal resonator from said certain resonance frequency value, said change being indicative of the identity and quantity of said foreign material.
The input and output of the sensor is connectable to a control means operable for actuating the at least one crystal resonator and measuring the change in the resonance frequency, to generate measured data representative of the identity and quantity of said foreign material.
The metal electrodes used in the sensor may comprise a metal selected from Au, Pt and Al, with Au being the most preferred metal. The modified surface region may include the surface of the membrane region, the surface of the respective electrode, or both.
The control means comprises an actuator utility (oscillator and one or more switches depending on the number of crystal resonators) and a detector utility. The actuator utility actuates said at least one crystal resonator to put it in operation. The detector utility comprises an electronic circuit for detecting the frequency of the at least one crystal resonator to enable measurement of said change.
According to yet another aspect of the present invention, there is provided a piezoelectric sensor device for identifying at least one foreign material from environment, the device comprising:
(i) a sensor including a piezoelectric crystal element having at least one piezoelectric crystal resonator in the form of an inverted mesa structure defining a sensing membrane-like region and being characterized by a certain resonance frequency value, and electrodes formed on opposite sides of said sensing membrane-like region, said at least one crystal resonator having a surface region modified with molecules capable of interacting with a foreign material of the environment to yield a reaction product that effects a change in the resonance frequency of said at least one crystal resonator from said certain resonance frequency value, said change being indicative of the identity and quantity of said foreign material; and
(ii) a control means operable for actuating said at least one crystal resonator, measuring the change in the resonance frequency of said at least one crystal resonator, and generating measured data representative of the identity and quantity of said foreign material.
Preferably, the crystal element comprises an array of spaced-apart crystal resonators, each in the form of the inverted mesa structure formed with a pair of electrodes at opposite surfaces of the membrane-like region, thereby defining an array of sensing regions affectable by the environment. The surface regions of different crystal resonators are modified with different reactive molecules, thereby enabling the detection of various foreign materials contained in the environment. Au electrodes, for example, are suitable for modification with sulfur containing molecules. When the quartz membrane region participates in the sensing operation, then the quartz may be modified by different functional groups, such as, for example, silanes.
The crystal element may comprise the so-called xe2x80x9creferencexe2x80x9d or xe2x80x9cpassivexe2x80x9d crystal resonator (without coating) which is screened from the environment. The change of the resonance frequency of the xe2x80x9cactivexe2x80x9d crystal resonator caused by the foreign material is thus determined as a difference between the resonance frequencies of the active and passive elements. Alternatively, such reference data (or the certain value of the resonance frequency characterizing the crystal resonator) may be stored in a memory of the control means.
Preferably, the crystal resonators are equally distanced from the actuator utility. This may be implemented by arranging the resonators (wherein one of the crystal resonators may be the reference or passive element) in spaced-apart relationship along a circular path and placing the actuator utility either in the center of the circle or at any other location at the central axis of the circle. Thus, the crystal element may comprise a disc carrying the circular array of the equally spaced piezoelectric crystal resonators, and the actuator utility located on the central axis of the disc. It should be understood that the entire disc may be made of a piezoelectric crystal, in which case the crystal is patterned to define the circular array of spaced-apart inverted mesa structures (i.e., the circular army of membrane-like regions defining the sensing regions). Alternatively, such inverted mesa structures of piezoelectric crystal resonators may be mounted on a disc made of any other suitable material.
According to yet another aspect of the present invention, there is provided a piezoelectric sensor device for identifying at least one foreign material from environment the sensor device comprising:
an array of spaced-apart crystal resonators, each crystal resonator being in the form of an inverted mesa structure having a membrane-like region and being characterized by a certain resonance frequency value, each of the crystal resonators being excitable by the environment to cause a change in the resonance frequency thereof from said certain resonance frequency value; and
an actuator utility for operating the crystal resonators, said actuator utility being equally spaced from each of the crystal resonators.
The modification of the surface region of the crystal resonator may be achieved by two alternative techniques: (1) construction of organized, self assembled monolayers (SAM); or (2) formation of polymeric layer.
The organized, self-assembled monolayer (SAM) consists of receptor compounds comprising a linker that connects said compound to the surface of said substrate, an optional spacer, a structural element and an active head group.
Relating to the formation of a polymeric layer, the preferred technology for forming a polymer layer in a controlled manner is by electropolymerization
It should be noted that the device of the present invention could be used in gaseous medium, liquid medium or a combination thereof, thus acting as an electronic nose and/or as an electronic tongue. An electronic tongue is a device similar to the electronic nose, but capable of operating in a liquid medium, and enables the analysis of solutes in a solution. Such a combined device provides for simultaneous analysis of a solution and it""s vapors, providing a complete picture of the detected material.